Mueller's stellar public service reputation preceded him, although his performance in Wednesday's hearings revealed to the public the truth about the probe, Trump claimed Thursday on "Tucker Carlson Tonight."
"I think the American people finally saw it, that this man -- who was the lead of the largest investigation in American history, really the largest hoax at this point -- he didn't have a fundamental understanding of not only where it began, but what was even involved," he said.
"Ultimately, I think the American public saw that he was put in there as a figurehead because he was the former leader of the FBI -- he was a decorated Marine and that he would do what's right."
Trump Jr. said the 74-year-old Mueller hired a handful of lawyers to engineer the probe day-to-day, many of whom opposed his father, President Trump.
"This guy put in 19 leftist, Hillary Clinton-donor lawyers," the Trump Organization executive said.
"He had no idea who was running the ship. The inmates were running the asylum. Those inmates were the Democratic Party."
Regarding the probe itself, Trump claimed it was a set up to undermine his father and stymie his administration.
"This was a hoax set up by the Democrats from moment number one," he said.
"This investigation wasn't about trying to figure out what happened, it was trying to back into the result that they wanted. They couldn't do that because there was nothing there."
Trump Jr. also referenced a key exchange between the former special counsel and Rep. Steve Chabot, R-Ohio, a member of the House Judiciary Committee.
"You referenced 'the firm in that produced the Steele reporting.' The name of that firm was Fusion GPS. Is that correct?" Chabot asked.
In his interview with Carlson, Trump remarked, "I see a congressman ask, 'let's talk about Fusion GPS' -- 'I've never heard of it'."
"That'd be like a builder talking about not understanding what a foundation is. To not even have a finite understanding of how this all started tells the American public everything they need to know," he said.
Trump added he believed the Democrats are doing whatever they can to "destroy" the elder Trump's presidency.